20 Best Outdoor Vacations for Firefighters

Living up to its name, this Kentucky attraction is the largest cave in the world, stretching 350 miles long and plunging 379 feet deep. Even its runner up, Ukraine’s Optimisticeskaya cave, is just a quarter of Mammoth’s length.

Take a discovery tour and explore the colorful stalactites and stalagmites, underground rivers and giant vertical shafts. And the natural wonder doesn’t end there. You’ll also get to see some of the world’s weirdest creatures, including eyeless fish, white spiders and blind beetles. Tip: If you’re not claustrophobic, or afraid of the dark, try the Spelunking tour — you’ll get a more authentic experience and see parts of the cave not normally accessible to the public.